1. Field of the Invention
The present invention relates to a method and apparatus for preparing a rare earth-activated barium fluorohalide based phosphor useful as a stimulable phosphor for a radiation image conversion panel.
2. Description of the Related Art
A radiation image recording and reproducing method using a stimulable phosphor (which will be hereinafter referred to simply as “phosphor” on occasion) is known, which can replace conventional radiography. This method utilizes a radiation image conversion panel containing a stimulable phosphor (that is, an accumulative phosphor sheet). In this method, radiation transmitted through a subject or emitted from an object to be examined is absorbed into the stimulable phosphor of the above-mentioned panel, and the stimulable phosphor is excited by an electromagnetic wave (excitation light) such as visible light or infrared light in a time sequential manner so as to allow radiation energy accumulated in the stimulable phosphor to emit as fluorescent emission (stimulable emission). Subsequently, an electric signal is obtained by photoelectrically reading the fluorescence, and based on the obtained electric signal, a radiation image of the subject or object to be examined is reproduced as a visible image. The panel for which the reading has been completed is made ready for subsequent photographing after remaining images have been deleted therefrom. That is, the radiation image conversion panel can be used repeatedly.
The above-described radiation image recording and reproducing method has an advantage in that a radiation image having much information can be obtained using a radiation dosage much smaller than in conventional radiography using a combination of a radiographic film and an intensifying screen. Further, a radiographic film is consumed each time an image is produced in conventional radiography. In contrast, a radiation image conversion panel in the radiation image recording and reproducing method is advantageous from the standpoint of resource protection and economic efficiency due to reusability.
The stimulable phosphor is a phosphor which exhibits stimulable emission when excitation light is irradiated thereon after having being irradiated with radiation. For practical use, a phosphor which emits stimulable emission in a wavelength ranging from 300 to 500 nm by excitation light in a wavelength ranging from 400 to 900 nm is generally employed. An example of the stimulable phosphor conventionally used in radiation image conversion panels, is a rare earth-activated barium fluorohalide based phosphor.
The rare earth-activated barium fluorohalide based phosphor is generally prepared by the following method.
First, a mixture of phosphor materials is prepared by homogeneously mixing phosphor materials in a dried state (a dry process) or by homogeneously mixing phosphor materials to form a slurry and thereafter drying the slurry (a wet process).
Subsequently, the obtained mixture of phosphor materials is ordinarily calcined at a temperature close to a melting point of a host crystal (for example, BaFX, or the like) in a neutral or weakly oxidizing atmosphere substantially under atmospheric pressure over several hours (calcining step). The obtained calcined product may be further calcined if desired. Due to the calcining step, the host crystal of the phosphor grows, and simultaneously, activator elements (Eu and the like) are diffused in the host crystal. Further, an F+-center which forms a central source of stimulation is also generated. Accordingly, the calcining step is an important step which exerts an influence on the light emission characteristics of the phosphor.
The calcined product thus obtained is cooled, and if necessary, subjected to washing, classification, and the like, to thereby form a phosphor.
After the calcining, the calcined product is generally cooled in a calcining furnace in a state of being placed therein, or cooled after being taken out of the calcining furnace. However, if the calcined product is cooled in the calcining furnace, an atmosphere temperature in the calcining furnace is high and the temperature of each of various members in the calcining furnace is also high. Therefore, a cooling rate of the calcined product cannot be increased so much. Particularly, even when rapid quenching is required for adjustment of the characteristics of phosphors, there is a limit to the cooling rate, which is disadvantageous to the adjustment of the characteristics of phosphors.
On the other hand, if the calcined product is cooled after being taken out of the calcining furnace, the cooling rate can be further increased. However, the state of an atmosphere during the cooling cannot be adjusted. The state of an atmosphere during the cooling becomes an important factor for the adjustment of the characteristics of phosphors.